Gravitational Wave Backgrounds

Gravitational wave background arises as a superposition of many uncorrelated gravitational wave sources of either cosmological or astrophysical origin. For each individually observed merger of a binary black hole or a binary neutron star system, there are many more that remain undetected as they are too far and too faint. This ensemble of compact binary mergers, however, gives rise to a gravitational wave background that carries information about the distant population of binaries, complementing the individually detected nearby ones. Furthermore, gravitational wave background is expected to be formed in the early universe through processes such as inflation, phase transitions, or cosmic strings dynamics. Detection of the early universe background would, therefore, provide unique information about particle physics of very high energies that may be unreachable in laboratories. With improved sensitivity in the upcoming observation runs of terrestrial gravitational-wave detectors Advanced LIGO, Advanced Virgo, and KAGRA, some of the astrophysical and cosmological models of gravitational wave background will be within reach. I will review the landscape of gravitational wave background models and I will discuss the prospects for detecting the background in upcoming observations. I will also discuss complementarity of these observations with other experiments and observations, such as future collider experiments and electromagnetic observations of structure formation.